The present invention relates to a developing unit for developing an electrostatic latent image using two-component developer in an image forming apparatus such as an electrophotographic copier.
In the conventional electrophotographic copier, a magnetic brush development type developing device is used to which two-component developer is applied. This developing device includes a developing sleeve, which functions as a cylindrical rotary conveyer to convey developer, and a magnetic roller composed of a magnetic body having a plurality of magnetic poles is provided inside the developing sleeve. Magnetic carrier particles on which toner particles are deposited are held on a surface of the developing sleeve, so that the carrier particles are conveyed to the development region by the developing sleeve.
In general, two-component developer is composed of magnetic carrier particles, the average particle size of which is several tens .mu.m to several hundreds .mu.m, and non-magnetic toner particles, the average particle size of which is approximately 10 .mu.m. When the two-component developer is used for development, the following problems may be encountered. Toner and carrier particles are rough. Therefore, it is difficult to provide an image of high quality on which fine lines and points are reproduced with high fidelity, and further a difference in image density can not be reproduced accurately. Conventionally, they made every effort to obtain images of high quality by this developing method, for example, carrier particles are coated with resin, and the magnetic body assembled into the developer conveyer is improved. In spite of the effort they made, images of sufficiently high quality can not be provided yet. As a result, it is necessary to reduce the sizes of toner and carrier particles so that finer particles can be provided. However, when the average size of toner particles is reduced, specifically, when the average size of toner particles is reduced to not more than 20 .mu.m, particularly when the average size of toner particles is reduced to not more than 10 .mu.m, the following problems may be encountered.
(1) In the process of development, Van der Waals force affects toner particles relatively stronger than Coulomb force. Therefore, toner particles are strongly deposited on the image forming body. Accordingly, toner particles are deposited on the background of an image. As a result, fog is caused. In this case, even if a DC bias voltage is impressed upon the developer conveyer, it is difficult to prevent the occurrence of fog.
(2) Carrier particles are covered by toner particles more thickly. Therefore, it becomes difficult to conduct triboelectric charging control.
(3) When carrier particles are covered by toner particles more thickly, coagulation of toner tends to occur.
(4) When the size of carrier particles is reduced to be fine, carrier particles are also deposited on the electrostatic latent image portion on the image forming body. The reason is that the force generated by the action of the magnetic bias is lowered, so that the carrier particles are deposited on the image forming body together with the toner particles. Further, when the bias voltage is increased, carrier particles are deposited on the background on the image.
Reduction of the sizes of toner and carrier particles is disadvantageous as described above, and it is impossible to provide clear images. Accordingly, it is actually difficult to reduce the sizes of toner and carrier particles from the viewpoint of practical use.
In order to solve the above problems, Japanese Patent Publication Open to Public Inspection Nos. 346736/1993 and 175485/1994 disclose a developing method which will be described below. In the upstream of the developing region, a plate member having an electrode is provided. The plate member comes into contact with the developer conveyer. An oscillating electric field is formed between the electrode and the developer conveyer, and an oscillating electric field is also formed between the developer conveyer and the image forming body. In this case, the intensity of the former electric field is higher than that of the latter electric field. In this way, toner particles in the developer are formed into clouds.
However, the above control electrode method is disadvantageous as follows. In the case of development conducted between magnetic poles in which a magnetic pole is interposed between the image forming body and the developer conveyer at a position where the image forming body is located closest to the developer conveyer, bristles of developer in the development region are made to lay down. In addition to that, the bristles are further suppressed by the plate-shaped electrode provided on the upstream side, so that the bristles of developer become too dense. As a result, toner on the lower layer is difficult to be used for development, and it is necessary to impress a high development bias voltage in which DC and AC components are superimposed. As a result, blur of an image tends to occur due to the electric discharge conducted on the photoreceptor and electrode. In the prior art described above, a technique is disclosed, in which the inner magnet is rotated simultaneously when the developer conveyer is rotated. According to the above technique, the development property fluctuates. Specifically, the development property in the case where the developing region is interposed between the magnetic poles is different from the development property in the case where the development region is located on the magnetic pole. Accordingly, it is impossible to stably provide images of uniform density.